1. Field of the Invention
The present invention relates to a catalyst assembly comprising a catalyst supported on a substrate and, especially, it relates to a catalyst assembly suitable for purifying an automobile exhaust gas.
2. Description of the Related Art
For example, a catalyst assembly for purifying an automobile exhaust gas comprising a cordierite carrier as a substrate and a noble metal as a catalyst supported on the carrier is known but, as the bonding strength to a noble metal is weak, the cordierite in itself cannot support a sufficient amount of a noble metal.
Therefore, conventionally, γ-alumina having a large specific surface area is supported, to a thickness of tens of micrometers, on a surface of cordierite, and catalyst particles are supported on the γ-alumina by the use of physical adsorption on this large surface area. Sometimes only particles having a catalytic function are supported, and sometimes a co-catalyst or particles having an occlusion function may be also supported in addition to catalyst particles as in a three-way catalyst for automobiles or a NOx occlusion reduction catalyst.
When a catalyst is supported on γ-alumina, the catalyst is supported not only on a surface of γ-alumina layer but also inside porous γ-alumina particles and between the particles. In this case, a catalyst supported on the surface works well, but there is a problem that a catalyst inside the γ-alumina particle does not work well because diffusion of a reactive gas to the inside of the particle is slow.
For this reason, it was necessary to support much more catalyst than a necessary minimum amount in order to attain the required purification.
When supporting two or more kinds of particles having different functions, such as a catalytic function and a co-catalytic function, it is preferable that the particles are adjacent to each other. However, if a catalyst particle is supported on γ-alumina, as γ-alumina exists also between each particle, there was a problem that the particles are adjacent to each other with a low probability and do not function sufficiently.
As there are problems that an amount of used catalyst was too high, a catalyst did not function sufficiently and so on, it is desired to use a catalyst more efficiently and make a catalytic function more efficient.